Sound in horror films

Sounds in horror films are the most important aspect as it creates a specific mood and atmosphere for certain scenes in the film. By making use of sound this adds more suspense to the scene and promotes even more emotions to the audience.

There are two types of sound, diegetic and non-digestic.
Diegetic - Any sound presented as originated from source within the film's world. Digetic sound can be either on screen or off screen depending on whatever its source is within the frame or outside the frame. 
Non-diegetic - Sound whose source is neither visible on the screen nor has been implied to be present in the action. It is represented as coming from a source outside of the story space. It can be:  

• narrator's commentary
• sound effects which is added for the dramatic effect
• mood music

Some of the common types of sounds are:
- Isolated instruments instead of a big orchestra are often used before an important event occurs to promote the vulnerability of the character and audience. 
- Long notes tend to create more suspense rather than short and sharp notes. This is because they portray the shock of characters.
- The use of long notes that suddenly change has an impact on the viewers emotions as they will become tense and scared immediately.

An example of fast, horror music:

Voices in horror films should reflect fear, tensions and panic. The voices are generally low and mysterious where as if the voices of the antagonist sounded friendly, the audience would not be very scared. Also, by using a deep voice, it portrays as sense of intimidation and power.

The sound effects used can be classic sounds which connote the horror genre such as owls, bats, crows, church bells etc. Echoed voices can also be used to give the audience a sense that a ghost or spirit is present.      https://www.youtube.com/watch?v=NHJZnvYtruM